How do I fix this garage? It's a concrete/rebar cube. The man who built it didn't insulate the roof (flat concrete roof) so the rebar got rusty and the inside layer of the concrete is crumbly. Now the roof has isolation layer, though the guy who added that didn't think to slant it outwards so the water still pools on roof. Its up in mountain and I'm poor so no option to get heavy machinery and demolish/rebuild

How do I stop this from breaking more? Is it possible to remove crumbly pieces, clean rust off rebar, spray rebar with something, then slap on it a new layer of concrete?

>>2921788 (OP)
There is very little you can do.
One thing that might help is to try and clean off some of the rust and apply as much zinc metal to the rebar as possible. It has to be in direct contact with the steel.
You could (painstakingly) electroplate it on, for example using a fiberglass wick and zinc chloride?? with a current source.
After thar, you would probably want to place zinc strips along the exposed rebar and attach hem electrically. Maybe tiny screws or clips, or whatever.
That will delay the inevitable rusting I think.

>>2921788 (OP)
First, you should stop the water from penetrating your concrete roof.
Then, remove all the loose pieces of concrete and remove as much rust from the rebar as you can.
Then, from the website of concrete companies, you can find the appropriate mortar for this kind of job. Yes, mortars for that job exist. They have improved adhesion